Gas chromatography is a process by which one or more compounds from a chemical mixture may be separated and identified. A carrier gas, for example, an inert gas such as nitrogen or helium, flows through a tube known as a column. Large columns may have inner diameters between about 3 mm and about 8 mm and lengths between about 1 meter and about 3 meters. Capillary columns may have inner diameters between about 0.05 mm and about 1 mm and may be 100 meters or more in length. The large column may be packed with an inert packing medium coated with an active substance that interacts with compounds in the chemical mixture being analyzed. Capillary columns are preferably coated on their inner surface with the active substance.
A sample of the chemical mixture to be analyzed is injected into the column. As the sample is swept through the column with the carrier gas, the different compounds, each one having a different affinity for the active substance lining the column or coating the packing medium, move through the column at different speeds. Those compounds having greater affinity for the active substance move more slowly through the column than those having less affinity, and this speed differential results in the compounds being separated from one another as they pass through and exit the column.
The carrier gas with the separated compounds exits the column and passes through a detector, which identifies the molecules. Various types of detectors may be used, including a thermal conductivity detector, a flame ionization detector, electron capture detector, flame photometric detector, photo-ionization detector and a Hall electrolytic conductivity detector. A two dimensional plot of the detector measurements against elution time or volume, known as a chromatogram, may be made, and from the chromatogram or the digital representation thereof the compounds may be identified.
Injection of the sample chemical mixture into the column is effected using a sample inlet assembly. The sample inlet assembly has an injection port that receives a syringe for injecting the sample into the inlet assembly. The inlet assembly is connected to the column with a seal that provides a fluid tight joint between the relatively large diameter of the inlet assembly and the small diameter of the capillary column.